The present invention relates to a photomultiplier tube comprising a photocathode, an input electrode and an electron multiplier with stacked dynodes.
The invention may be applied particularly advantageously in the field of photomultiplier tubes having electron multipliers with stacked dynodes. "Electron multipliers with stacked dynodes" is to be understood to mean all the multiplier devices having a laminated structure, for example, the multipliers of the "sheet" type (see, for example, French Pat. Specification No. 2,549,288) or also the multipliers having shutter dynodes in which each dynode is constituted by parallel slats inclined with respect to the axis of the multiplier.
A general technical problem which presents itself in any photomultiplier tube is to ensure a collection of photoelectrons issued from the photocathode which is as large as possible. In the case of the tubes comprising a multiplier device with stacked dynodes, this general technical problem is coupled with another problem, namely that of coupling the first dynode to the multiplier device in such a manner that the secondary electrons emitted by the photocathode can reach the multiplier device with stacked dynodes with low losses.
A solution to this double technical problem is given, for example, in French Pat. Specification No. 2,549,288 (FIG. 12) which describes a photomultiplier tube as mentioned in the opening paragraph and the first dynode of which is cylindrical, having generatrices orthogonal to the axis of the tube. In the said known tube the coupling between the first dynode and the electron multiplier device with stacked dynodes of the "sheet" type is realised by placing the multiplier at the output of the first dynode, the axis of the "sheet" multiplier being provided perpendicularly to the axis of the tube. So in this configuration the multiplier device offers the greatest possible collecting section for secondary electrons emitted by the first dynode, hence a good collection efficiency.
However, the photomultiplier tube known from the prior art has the disadvantage of a comparatively large lateral space due mainly to the fact that, taking into account the rather important dimensions of the first dynode and the dispersion of the secondary electrons emitted by the latter, the "sheet" multiplier may not be placed in the immediate proximity of the first dynode. Moreover, an exit is needed towards the rear of the multiplier for the output connections, which contributes to augmenting the dimensions of the sleeve serving as envelope of the tube.
Moreover, the known tube also has the disadvantage of a considerable longitudinal space, connected with the necessity of having to provide at a sufficient distance between the photocathode and the first dynode, to permit the focussing input electrode to ensure its function of concentrating the photoelectrons on the first dynode.